Atomic Radius Trend
While we learn about the behaviour of a compound or an atom, the size of the atoms plays an essential role. One of the ways in which we can express the size of an atom is with the help of atomic radius. It helps us in understanding why some of the molecules fit in together and the rest contain parts which get crowded under different conditions. The atomic size is defined by its orbital edge. However, these orbital boundaries are quite fuzzy and tend to vary in different conditions. For standardizing the measurement of the atomic radius, the distance between the nuclei of two similar atoms that are bonded together is measured. We can, therefore, define the atomic radius of elements as half of the distance between the nuclei of similar atoms which are bonded together.
Today, we will learn about what is atomic radius periodic table, the atomic radius definition, what is the atomic radius trend, what is atomic size periodic table, and the atomic size trend.
Atomic Radius Definition
Let us now define the atomic radius.
The atomic radius of an element refers to the measure of the size of the element’s atoms, which is typically the mean distance from the nucleus centre to the boundary of its surrounding shells of the electrons. However, since the boundary is not well-defined, there are several non-equivalent definitions of the atomic radius. There are three types of atomic radii which are Van der Waals radius, covalent radius and ionic radius.
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Measurement of Atomic Radius
Let us learn how the atomic radius is measured and what is its unit.
Atomic radii are measured for elements. The units for measuring the atomic radii are picometers, which is equal to 10−12 meters. Consider, for, example, the internuclear distance between two hydrogen atoms in the H2 molecule is known to be 74 pm. Hence, the atomic radius of the hydrogen atom is 74/2 = 37 pm
Atomic Size Trend
Let us learn about what is the trend in atomic radii down a group.
When we move down the group or across the row or column in the periodic table, we would observe a lot many trends in the elements, both physical and chemical. Consider, for example, while moving down the group of the non-metals, the reactivity of the elements tends to decrease, whereas, it tends to increase when we move down the group of the representative metals.
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When we combine two atoms, we can then estimate their atomic size when we check the distance between the two atoms. The other method through which we can measure the atomic size of a given non-metallic element is by the formation of a single covalent bond between the two atoms and then check the distance between the two atoms. The radius which is found by this method is called the covalent radii of the element. However, in the case of a metal, it is known as a metallic radius. It is defined as one half of the distance between the nuclei of the two adjoining metal ions that are joined by a metallic bond.
The atomic radius of an atom is measured with the help of X-ray or several other spectroscopy methods. The atomic radii of the elements tend to vary in the periodic table but a fixed manner. We can explain this trend when we consider the nuclear charge and the energy level.
Typically, the atomic radius tends to decrease as we move from the left to right in a period and it tends to increase when we move downwards in a group. The reason being that in periods, the valence electrons tend to lie in the same outermost shell. The atomic number tends to increase within the same period when we move from the left towards the right which tends to increase the overall effective nuclear charge. The increase in the attractive forces, in turn, reduces the atomic radius of the elements.
You know that the force of attraction between the protons and electrons tend to play a very important role in the increasing or decreasing pattern of the atomic radius.
FAQs on Atomic Size and Atomic Radius
1. What is atomic radius and how is it precisely measured?
Atomic radius is a measure of the size of an atom, typically defined as half the distance between the nuclei of two identical, bonded atoms. Since the electron cloud boundary is not sharp, direct measurement is impossible. Instead, it is determined using X-ray diffraction or spectroscopic methods on molecules or crystals. The value is usually expressed in picometers (pm), where 1 pm = 10⁻¹² m. For example, the distance between two chlorine nuclei in a Cl₂ molecule is 198 pm, so the covalent radius of a chlorine atom is 99 pm.
2. What is the fundamental difference between atomic size and atomic radius?
While often used interchangeably, there is a technical difference. Atomic size is a general, qualitative term describing how large an atom is. Atomic radius is the specific, quantitative measurement used to define that size. In essence, atomic radius is the scientific value we assign to an atom's size, based on measurements like covalent, metallic, or Van der Waals radius.
3. Why does atomic size decrease when moving from left to right across a period in the periodic table?
Atomic size decreases across a period because, while electrons are added to the same principal energy level (shell), the number of protons in the nucleus also increases. This leads to a higher effective nuclear charge—a stronger pull from the nucleus on each electron. This increased attraction pulls the electron shells closer to the nucleus, resulting in a smaller atomic radius.
4. Why does atomic size increase when moving down a group in the periodic table?
Moving down a group, the atomic size increases due to two main factors as per the CBSE syllabus:
- Addition of a new electron shell: Each element down a group has an additional principal energy shell, placing the outermost electrons further from the nucleus.
- Increased shielding effect: The inner-shell electrons shield the outermost electrons from the full attractive force of the nucleus. This effect increases with more shells, lessening the nucleus's pull on the valence electrons.
5. What are the different types of atomic radii defined in Chemistry?
There are three primary types of atomic radii, each used in different contexts:
- Covalent Radius: Half the internuclear distance between two identical atoms joined by a single covalent bond (e.g., in H₂ or Cl₂). It is used for non-metallic elements.
- Metallic Radius: Half the internuclear distance between two adjacent metal ions in a metallic crystal lattice. It is used for metallic elements.
- Van der Waals Radius: Half the distance between the nuclei of two non-bonded, adjacent atoms in their solid state (e.g., between two separate Cl₂ molecules). It is used for noble gases and for comparing distances between non-bonded atoms.
6. How does the shielding effect influence the atomic radius of an element?
The shielding effect (or screening effect) is the repulsion between the inner-shell electrons and the outermost (valence) electrons. This repulsion effectively reduces the positive charge from the nucleus that the valence electrons 'feel'. A stronger shielding effect means the valence electrons are held less tightly and are farther from the nucleus, which results in a larger atomic radius. This is a primary reason why atomic size increases down a group.
7. How does the radius of a cation or anion compare to its neutral parent atom?
The comparison is a core concept in periodicity:
- A cation (positive ion) is always smaller than its parent atom. This is because it is formed by losing one or more electrons, which reduces electron-electron repulsion and increases the effective nuclear charge pulling on the remaining electrons.
- An anion (negative ion) is always larger than its parent atom. This is because the addition of one or more electrons increases electron-electron repulsion and spreads the electron cloud out, making the ion larger.
8. Are there any notable exceptions to the general trends in atomic radius in the periodic table?
Yes, there are important exceptions. A key one is the atomic size of noble gases. For instance, Neon's atomic radius is larger than that of Fluorine, which is contrary to the general trend across a period. This is because the radius for noble gases is measured as a Van der Waals radius (since they don't form bonds), which is inherently larger than the covalent radius used for halogens like Fluorine. Another exception occurs in the transition metals due to the poor shielding by d-electrons, causing the atomic size to remain relatively constant across the series.
